Study On Airflow And Resistance Characteristics Of A 3d Canopy Structure

With the rapid development of urbanization and growing environmental pollution, more and more people pay close attention to the quality of the air. Atmospheric particulate has become environmental priority pollutants in the air of many cities, and they damage human healty strongly.In order to alleviate air pollution problem, we find a practical and effective method that urban green space has good effect in absorption of dust in the air. Trees have the function of reducing dust, conserving water and soil resources, breaking wind and fixing sands, regulating the climate, purifying air and so on. Management strategies to reduce wind erosion have incorporated the use of windbreaks, shelterbelts and structural barriers that result in a reduction of wind speeds in the lee. Planting or maintaining natural vegetation cover at sufficient canopy densities can eliminate the likelihood of wind erosion.So the ability of plant reducing dust has become the important indicator of plant selection of urban green space.Therefore it forced us to study the law of convection and diffusion of the mechanism of reducing dust.In order to obtain the flow field and concentration field of the canopy moeld and study on the realisitic single canopy model.The main research work and conclusions are as follows:(1)Visual basic application(VBA) produces a novel 3D artifical architecture of canopy that contains a hard trunk, branches and artificial leaves. Optical porosity( op) is measured by digitized scanning method, and measures of three-dimensional volumetric pofosity( vp) are taken using a simple water displacement gechnique,then we will get. Computational fluid dynamics(CFD) was used to investigate the flow characteristics and flow resistance through threedimensional microcosmic vegetation canopies with different porosities and various velocities. Optical porosity and volumetric porosity were used to analyze the complicated morphology of vegetation canopy, and analyze the influences of these morphology of vegetation canopy on flow resistance. The relationship between the drag coefficient and the optical porosity can be approximately predicted as a quadratic polynomial, and a model expression was presented by using simulation results. The model expression was validated using experimental results in a wind tunnel with tree branches. Based on the comparison of the drag coefficient data obtained using the field experiments in real environment, it provides a basis for predicting the real drag coefficient.(2)In order to analyze the effect of leaves and the other parts of the canopy on airflow, we investigate flow properties within the canopies in diffierent with the method of combining the numberical simulation and experimental test. Compared with the traditional modeling approaches, adding source terms to the governing equations to represent the combined effect of branches and leaves in the computational domain. Simulations are made with a full closure model(FCM) and the microcosmic model(MM). Canopies used in the simulations had three leaf area density():1.36, 2.26, 3.17, 4.07 and 4.53 m-1. The objectives of this paper are to analyze the contour of velocity and(Turbulence kinetic energy) of two models in different, compare the simulation results with experimental data/other works and investigate the real effects of the canopy on the airflow distribution. Results are encouraging, compared with the FCM, the MM profiles of average velocity() and turbulent kinetic energy() qualitatively agree better with other works. Therefore the model and method are recommended for future use in simulating turbulent flows in forest canopies.(3)Discrete phase model(DPM) was used to simulate the concentration field of particulate pollutant through three-dimensional single canopy model. The relationship between the deposition velocity and canopy structure parameters is analyzed. It is discussed that the relationship of collection dfficiency of the single canopy model and the wind speed, leaf area density and particle size. In order to verify the correctness of simulation data,we contrast it with the classical theory of other works. In the end, the expression of the efficiency of a single canopy is obtained by regression analysis.